The steps to get this Wpf .Net 4.5 UI working are:

1. Add a reference to your Core PCL project
2. Create a Setup.cs file
3. Change App.Xaml.cs so that it creates a 'new Setup(RootFrame)' and starts the app:

    public partial class App : Application
    {
        private bool _setupComplete;

        private void DoSetup()
        {
            var presenter = new MvxSimpleWpfViewPresenter(MainWindow);

            var setup = new Setup(Dispatcher, presenter);
            setup.Initialize();

            var start = Mvx.Resolve<IMvxAppStart>();
            start.Start();

            _setupComplete = true;
        }

        protected override void OnActivated(EventArgs e)
        {
            if (!_setupComplete)
                DoSetup();

            base.OnActivated(e);
        }
    }

4. Add a views folder and a View - xaml.cs and .xaml based on UserControl
  - Change the inheritance of this user control to MvxWpfView (in both Xaml and CS)
  - Add some content for your Xaml - e.g. <TextBlock Grid.Row="1" Text="{Binding Hello}" />
